Sagan crashed heavily on the descent from the Col de Val Louron-Azet on stage 17, crossing the line at the end of the day with large tears to his jersey and shorts and blood running down his left arm and leg.
The world champion, who already had an unassailable lead in the points classification, looked in good shape on Thursday's flat stage but was dropped early on the Col d'Aspin well before the halfway point of stage 19.
With team-mates Maciej Bodnar and Daniel Oss for company, Sagan looked inconsiderable discomfort as he made his way up the first category climb, constantly shifting in and out of the saddle while pouring entire bottles of water over his head in an attempt to cool down.
Sagan crossed the top of the Aspin more than eight minutes down on the front of the race and four minutes behind the peloton, but was able to regain contact with a larger gruppetto on the descent that followed where he remained for the rest of the stage.
As Primož Roglič (LottoNL-Jumbo) won the stage in a time of 5-28-17, the time limit was set at just under 46 minutes, which Sagan's group comfortably made as they came home 38-23 down with Sagan sitting alongside the long-suffering Lawson Craddock (EF Education-First Drapac).
Last place on the stage went to Craddock's team-mate Taylor Phinney, who finished alone two minutes later with a large bandage across his face and blood running down his chin and onto his jersey.
Getting through the final mountain stage means that Sagan is now almost certain to claim a record-equalling sixth green jersey, needing only to stay upright through the final two stages to match Erik Zabel's record.
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